I tried ubuntu-18.04-preinstalled-server-armhf+raspi3.img and ubuntu-mate-18.04.2-beta1-desktop-armhf+raspi-ext4.img using a 64 GB SanDisk SDXC. On the server, I used tasksel to install, first, Lubuntu, and then, Mate. Both times, I was able to install the entire desktop and configure it, but on reboot, I got the message "Did not find cmdline Flattened Device Tree.", which was followed by numerous unsuccessful network connect attempts, then finally an address acquired by DHCP, and then a U-Boot prompt. I was not able to find on the internet any way to recover function of the installation thereafter. The Mate desktop image file installation simply would not boot to either a GUI or a command prompt, though I found by inserting the micro SD card into another machine that the FAT16 and EXT4 partitions were installed and the EXT4 partition was expanded to fill the SD card. I finally succeeded in installing Raspex (persistent through reboots) from raspex-ubuntu-18.10-lxde-4400mb-181015.img, which is, of course, not Mate, but rather a version of LXDE using Ubuntu 18.10. I am leaving this primarily as feedback for developers of the R Pi server and Mate desktop versions, but I would be interested in any available insight for installing desktops running Ubuntu 18.04 or 18.10 on the R Pi. I am wondering if I should try using tasksel on my current installation to install Mate, but I would like some guidance, if anyone knows anything about this.